1. Field of the Invention
The present invention relates to a power series predistorter for compensating for the distortion component produced by a power amplifier, and to a power series predistorter control method.
2. Description of the Related Art
A nonlinear distortion compensation method for a microwave transmit amplifier is a power series predistortion method. FIG. 1 shows a functional block diagram of a conventional power series predistorter 800. The power series predistorter 800 includes a delay path 825 for delaying a signal with a delay line 820, a distortion generation path 835 having a third-order distortion generator 830 and a vector adjuster 840, a divider 810 for dividing the input signal between the delay path 825 and the distortion generation path 835, a combiner 850 for combining the output of the delay path 825 and the output of the distortion generation path 835, and a controller 880 for minimizing the distortion component produced by a power amplifier (microwave transmit amplifier) 860 by controlling the vector coefficients of the vector adjuster 840. A directional coupler 870 returns a part of the output signal of the power amplifier 860 to the controller 880 through a control path 875. A distortion generation path generating a fifth or higher odd-order distortion component may be connected in parallel to the distortion generation path 835.
A vector coefficient control method of the power series predistorter uses two equal-amplitude pilot signals, as indicated in Toshio Nojima, Yoshiharu Okamoto, Toni Oyama, “Predistortion Nonlinear Compensator for Microwave SSB-AM System,” IEICE Transactions, Vol. J67-B, No. 1, January 1984 (Non-Patent Document 1). The method of Non-Patent Document 1 executes the following processing. When the pilot signal is input to the power amplifier 860, an intermodulation distortion component produced in the power amplifier 860 generates a third-order intermodulation distortion component in the adjacent frequency of the pilot signal. The controller 880 detects the third-order intermodulation distortion component of the pilot signal from the output of the power amplifier 860 and controls the vector coefficients of the vector adjuster 840 in the distortion generation path so that the detected third-order intermodulation distortion component is minimized. The vector adjuster 840 includes a variable attenuation unit and a variable phase unit, which are not shown in the figure, and adjusts the amplitude component and the phase component, which are the vector coefficients, under the control of the controller 880.
The control algorithm of the controller 880 uses a perturbation method to follow temporal changes and temperature changes of the power series predistorter. In the perturbation method, the distortion components are measured when each of the specified vector coefficients is changed to a smaller value and a larger value, and the vector coefficient is specified again after a constant offset is applied in a direction in which the distortion component is reduced. The steps for setting the vector coefficients are repeated until the distortion components measured when each of the specified vector coefficients is changed to a smaller value and a larger value become equal.
FIG. 2 shows a functional block diagram of another conventional power series predistorter 900 indicated in Shinji Mizuta, Yasunori Suzuki, Shoichi Narahashi, Yasushi Yamao, “New Adjustment Method for Frequency Characteristics Compensator of the Digital Predistortion Linearizer Compensating Frequency-Dependent IMD with Continuous Spectrum,” C-2-15, Electronics Society Conference, IEICE, September 2005 (Non-Patent Document 2). The power series predistorter 900 includes a delay path 925 for delaying a signal with a delay memory 920; a distortion generation path 935 having a third-order distortion generator 830, a vector adjuster 840, and a frequency characteristic compensator 990; a divider 810 for dividing the input signal between the delay path 925 and the distortion generation path 935; a combiner 850 for combining the output of the delay path 925 and the output of the distortion generation path 935; and a controller 980 for minimizing the distortion component produced by a power amplifier (microwave transmit amplifier) by controlling the vector adjuster 840 and frequency characteristic compensator 990. The frequency characteristic compensator 990 includes an N-point FFT unit 991, N complex multipliers 992-1 to 992-N, and an N-point inverse FFT unit 993. The N-point FFT unit 991 applies serial-to-parallel conversion to N points of the input signal and then applies N-point discrete Fourier transform to the parallel-converted N points of the input signal. The N-point inverse FFT unit 993 applies N-point inverse discrete Fourier transform to the output signals from the N complex multipliers 992-1 to 992-N and then applies parallel-to-serial conversion to the N inverse-Fourier-transformed output signals with a parallel-to-serial converter to generate N points of the output signal. The controller 980 minimizes the distortion component produced by the power amplifier by controlling the vector coefficients of the vector adjuster 840 and the complex multiplier coefficients of each of the complex multipliers 992-1 to 992-N. This power series digital predistorter is also controlled by the perturbation method.
In mobile communication systems, a base station transmitter performs transmission output power control so as to reduce interference between base stations while not degrading the channel capacity. In the W-CDMA system, for example, the transmission output power is changed by 1 dB at control intervals of 0.625 ms. As described above, the power series predistorter must specify the vector coefficients of the vector adjuster and the like to linearize the nonlinearity of the power amplifier. Accordingly, the power series predistorter must follow the dynamic changes in transmission output power.
The perturbation method, which is one conventional control method, was originally adopted to follow temporal changes or temperature changes, which are very slow in comparison with the control intervals of the transmission output power. The conventional control method provided to follow the temporal changes or temperature changes may not be able to follow the dynamic changes in transmission output power, because the perturbation method uses a constant offset value. One method of increasing the control speed of the perturbation method is to increase the offset value, but the increased offset value may increase the setting error of the vector adjuster. A decreased offset value cannot increase the control speed.
The same problem occurs in initial setting or during a transmission signal burst (state including a period of absence of the transmission signal), and a high-speed convergence algorithm has been needed.
In a power series digital predistorter provided with the frequency characteristic compensator, the complex multiplier coefficients must be specified for N complex multipliers. As the number of complex multiplier coefficients increases, a longer control time is needed.
Accordingly, it is an object of the present invention to provide a power series predistorter that can quickly set the coefficients of a vector adjuster thereof and the complex multiplier coefficients of the frequency characteristic compensator thereof, and a power series predistorter control method of the power series predistorter. More specifically, an object of the present invention is to provide a power series predistorter that can calculate these coefficients with a small calculation cost and that can compensate for the distortion component with the coefficients, and a power series predistorter control method of the power series predistorter.